The invention relates to a method for regenerating the original data of a digitally coded video film and an apparatus for carrying out the method.
The invention is based on a method for regenerating the original data of a digitally coded video film of the generic type of the independent Claim 1. Apparatuses of this type have recently become known for example as a result of the commercially available DVD (Digital Versatile Disk) playback devices. These devices are designed in accordance with the so-called DVD Standard (Version 1.0). In this standard, reference is made to the so-called MPEG standards MPEG1 and MPEG2 with regard to the reproduction of video films. The MPEG2 standard is also known under the reference ISO/IEC 13818: 1994 xe2x80x9cInformation technologyxe2x80x94generic coding of moving pictures and associated audioxe2x80x9d. The MPEG1 standard is also known under the designation ISO/IEC 11172: 1993 xe2x80x9cInformation Technologyxe2x80x94coding of moving pictures and associated audio for digital storage media up to about 1.5 Mbit/sxe2x80x9d. The video coding type is defined by one of these two standards in the case of DVD. According to these standards, the individual pictures of a video film are converted into a coded digital data stream (bit stream). A wealth of information is provided in the bit stream for each picture. All the details of the digital bit stream cannot be described in detail here. Reference is made in this respect to the standards mentioned, from which the necessary details can be obtained.
Today""s DVD playback devices encounter the problem that they cannot always operate without any errors in everyday operation, one of the reasons being that on account of the drastic increase in the storage density on the DVD itself, error-free reading cannot be effected in every situation. Slight scratches as well as dust and grease spots can then very easily prevent satisfactory reading of information. The nature of the recording means that even when the scratch/spot is relatively small, relatively large areas may be affected on account of the high storage density, and are not correctly evaluated. This results in poor picture and sound quality from time to time, despite error concealment measures that are present. The invention will address a specific problem with regard to picture display, but not the decoding of audio information.
As will be explained in more detail below, the video information items are stored in so-called sectors on the DVD. Such a sector comprises 2048 bytes of digital video information. Since the data in such a sector are compressed to a very great extent and correspondingly little redundancy is present, it is often the case, in spite of the fact that only a few items of data of a sector were not able to be read correctly, that the entire sector is omitted as video information because the data can no longer be completely decoded when part of the information has been lost. Consequently, it can happen that entire sectors are omitted during the reproduction of a video film. The MPEG standards MPEG1 and MPEG2 provide virtually a hierarchical division of the video data according to their importance. There are data which are important for recovering the video information items of an entire sequence of pictures; there are other data, in turn, only for recovering the information of an individual picture; and again there are other data which are absolutely necessary for recovering information items of specific sections in a picture.
The present invention will consider in further detail particularly those data which apply to a so-called Group of Pictures. The digital data stream is therefore provided with a section, called xe2x80x9cGroup of Pictures Headerxe2x80x9d GOPH, which also contains important system time reference data. A system clock is fed these data and synchronizes practically all of the operations within the DVD playback device. The system clock in the DVD playback device is from time to time readjusted by this system time reference data, thereby ensuring synchronous operation between the video decoding operation and the video encoding operation. Thus, in the same way that there is a specific GOP header for a Group of Pictures, there is also a header, called xe2x80x9cPictures Headerxe2x80x9d, for an individual picture. This header contains inter alia a so-called temporal reference code (temporal_reference_code). This code consists of a 10-bit-wide number and in practice indicates the position of this picture within the Group of Pictures, which then finally governs when this picture is displayed within the Group of Pictures, that is to say the order of display of the pictures is indicated thereby. The individual pictures of the Group of Pictures are thus, as it were, consecutively numbered. However, since the pictures are scrambled together in a specific manner during the encoding operation, the coding order no longer corresponds to the display order. This is in turn due to the fact that in accordance with the MPEG1 and MPEG2 standards, three different picture types are provided and the pictures are accordingly coded differently. The so-called I pictures corresponding to intra-coded pictures are known. These pictures contain only information items of one picture and can therefore be decoded independently of other pictures. The so-called P pictures, corresponding to unidirectionally predicted pictures, are provided as a further picture type. These pictures have been predicted on the basis of a preceding I or P picture, with the result that their data volume is reduced. They are thus dependent on a preceding I or P picture, however. The B pictures, corresponding to bidirectionally predicted pictures, are provided as the third picture type. These pictures are dependent on two neighbouring I or P pictures. They are predicted from the neighbouring I or P pictures. The average value is formed and the difference from the original picture is calculated. Since only a fraction of video information items remain in the case of the difference, the B pictures are considerably reduced in terms of their data volume. The provision of such different picture types also necessitates the scrambling of the display order for the purposes of coding. Specifically, a B picture can only be coded when the preceding and succeeding I or P picture is present in coded form. Consequently, in the data stream the chronologically succeeding I or P picture will always appear first before the B pictures which are actually to be displayed chronologically earlier. The original display order can be recovered during the decoding operation using the concomitantly transmitted temporal reference codes. A problem arises whenever, on account of an error, a GOP header is omitted and suddenly pictures of another Group are decoded which have a smaller temporal reference code than pictures that have already been decoded and are located in a memory. Specifically, it can then happen that the pictures with the higher temporal reference code which are actually to be displayed earlier are only displayed after the pictures with the lower reference codes which are actually to be displayed later. This, of course, leads to an incorrect display of the pictures, which is clearly discernible to the viewer. Since the picture with the higher temporal reference code remains continually in a frame memory, the display order within the new Group of Pictures can continue to be scrambled. This leads to undesirable xe2x80x9cjitterxe2x80x9d of the picture, which is extremely disturbing. The effect will be explained in more detail below with reference to pictures.
The object of the invention is to prevent in particular the undesirable scrambling of the display order after an omission of a GOP header.
The object is achieved according to the invention by means of the method specified in the description of the preferred embodiment. According to the invention, namely, an independent time base is used to recover the display order in the DVD playback device. In this case, a few information items of the decoded data stream are buffer-stored over a specific period of time in order to ensure the independent time base. The particular advantage of these measures consists in the fact that scrambling of the display order after the omission of a GOP header does not arise.
The description of the preferred embodiment specifies advantageous information items which can be evaluated for the correct recovery of the display order.
The description of the preferred embodiment specifies an advantageous measure which can advantageously be used particularly when, in the coding operation, variable sequences of picture types are permitted and a defined sequence of picture types for the pictures to be coded does not have to be rigidly complied with.
The description of the preferred embodiment specifies an advantageous measure for a frequently selected sequence of picture types during a coding operation.
For an apparatus for carrying out the method according to the invention, it is very advantageous if a table is stored in which the information items for recovering the display order are stored for a number of decoded pictures and which is evaluated by a control unit in order to ensure the independent time base. Such a table can also be realized in a simple manner by software, with the result that the circuitry outlay remains low.
The measures in detailed in the description of the preferred embodiment are likewise advantageous since, using the counter readings of the claimed counters, it is easy to ascertain which type of picture a preceding picture was. Since the counter readings are a matter of simple numerical values, these can easily be compared with one another in order rapidly to obtain the picture type. This information combined with the information regarding whether or not the picture has already been released for display easily permit, given a joint consideration of a plurality of preceding pictures, a conclusion to be drawn regarding which picture must subsequently be released for display. According to the description of the preferred embodiment, the size of the table is advantageously adapted such that the picture sequence having the largest number of successive B pictures, including the I or P pictures adjoining these B pictures, can still just be called up in the table.